The mangrove forest of the Sundarbans provides important protection against storms and flooding for cities including Kolkata (Calcutta), India, on the Bay of Bengal. But global warming is simultaneously inundating already unstable mangroves and intensifying storms, with potentially devastating consequences for millions of people.1

Key Facts

The Sundarbans of India and Bangladesh is the world's largest mangrove forest.2 Accelerating sea-level rise due to global warming16,22,23 is likely to submerge the Sundarbans.20 This would eliminate the protection they provide against the region's increasingly intense tropical storms.5

By absorbing some of the force of wind and waves and serving as a flood barrier, mangroves can lessen the damage caused by cyclones and other storms.6,7,8

In early 2010, a disputed Sundarbans island disappeared under the rising waters of the Bay of Bengal.19 Scientists project that under a high emissions scenario, relative sea-level rise is likely to inundate most of the Sundarbans by mid-century, and could wipe them out by the end of the century.20,24

Without the mangroves of the Sundarbans to serve as a buffer, more frequent and intense storms are likely to pose a growing danger to the residents of Ganges basin including cities like Kolkata (Calcutta).3

Details

The people of the Ganges-Brahmaputra Delta—including the metropolis of Kolkata (Calcutta), India—depend on the mangroves of the Sundarbans for protection against storms and floods. As climate change destroys mangroves and worsens storms in the region, it puts lives and livelihoods at risk.

The Sundarbans is the world's largest mangrove forest. Designated as a United Nations World Heritage site in both India and Bangladesh,2 it covers nearly 4,000 square miles (10,000 square kilometers). The forest provides habitat for the Bengal tiger, as well as numerous other rare and endangered species of birds, reptiles, and aquatic mammals.2

Mangroves play a vital role in coastal ecosystems and food chains, by supporting communities of fish and shellfish.3,4 Mangroves are salt-tolerant trees and shrubs that help protect coastal areas from increasingly intense tropical storms, waves, and erosion.5,6,7,8 By serving as a flood barrier, they can reduce the damage caused by storms such as cyclones.6,7,8 Damage and erosion to Mangroves leave the coast increasingly exposed and therefore more vulnerable to storms. More than a quarter of a million people—60 percent of them in Bangladesh—died in tropical cyclones in the last two decades of the twentieth century.3

Densely populated coastal areas like the Ganges-Brahmaputra Delta are the most vulnerable to deadly storms.3,9 Scientists determined that intact and healthy mangroves in the Indian state of Orissa saved many lives in a 1999 cyclone originating from the Bay of Bengal.10 Another study found that the 2004 Indian Ocean tsunami caused significantly less damage in areas of southeastern India protected by mangroves and other forests, and simulations show that a dense belt of mangroves can dramatically reduce the peak pressure of a tsunami wave.11

However, in the last half of the twentieth century a substantial portion of the mangroves in South and South-East Asia were lost. This was largely due to human activities—including deforestation and large-scale conversion of mangroves to shrimp farming.3,5,12,13

Global warming compounds the dangers to the Sundarbans. These low-lying mangrove forests are highly susceptible to the effects of sea-level rise—including inundation of coastal areas, increased exposure to storm surges, increased coastal erosion, and rising salinity in ground and surface waters.3,5,14,15

During the twentieth century, global mean sea level rose at an average of 0.07 inches (1.8 millimeters) per year,16 but between 1993 and 2003, the average rate of sea-level rise nearly doubled to increase around 0.12 inches (3.1 millimeters) per year.17

Local sea-level rise of as much as 1 inch (25 millimeters) per year has been recorded in parts of the Ganges-Brahmaputra Delta.14,18 In early 2010, the rising waters of the Bay of Bengal claimed the Sundarbans island of New Moore/South Talpatti, the subject of a long territorial dispute between India and Bangladesh.19

What the Future Holds

Unless we make deep and swift cuts in our heat-trapping emissions, most of the Sundarbans may disappear underwater, and those that remain could be threatened by saltwater incursion.20

If we continue along a high heat-trapping emissions trajectory,21 global sea level is projected to increase as much as 23 inches (59 centimeters) over recent average levels by the end of this century.22 If, on the other hand, we make significant efforts to reduce emissions,21 sea level rise between now and the end of the century could be limited to around 15 inches (38 centimeters).22

Taking into account recent evidence of accelerating global sea-level rise16,23 as well as regional variations, scientists project that relative sea-level rise is likely to inundate most of the Sundarbans by mid-century, and could wipe them out by the end of the century.20,24

Sea-level rise and loss of the Sundarbans could have a devastating impact on the 500 million people of the Ganges basin.5 Tens of millions of people in low-lying areas of South Asia could be flooded annually.5,25 India and Bangladesh are particularly susceptible to increasing salinity of water resources, especially along the coast.5,14

For residents of cities like Kolkata, the greatest danger is likely to come from higher tides and more intense storms—with storm surges unchecked by the disappearing mangroves of the Sundarbans.3 As sea levels rise and storm patterns shift in the Bay of Bengal, scientists project increases in extreme water levels near Kolkata.3,26

The choices we make today could determine whether the ecologically rich Sundarbans stay on the map—affecting the very survival of people in Kolkata and throughout the Ganges basin.